In 1972 S. J. Singer and G. L. Nicolson (Science Vol. 175, p. 720) proposed that membranes of cells and cellular organelles in living organisms have a basic structure in common which may be described as follows:
Two layers of phospholipid molecules are arranged such that the hydrocarbon chains of said phospholipid molecules face each other and the polar ends of said phospholipid molecules face the intracellular or extracellular water. Interspersed between said phospholipid molecules are large protein molecules which traverse either or both of the lipid layers. The molecules of lipids and proteins are not chemically bonded to each other but can move laterally within the layers without interrupting continuity of said layers. The free movement of the proteins has been compared to "icebergs floating in a sea of water," and thus the term "Fluid Mosaic Membrane" has been adopted for this concept of membrane structure. Said concept is now generally accepted by the scientific community. The unique structure of the phospholipid molecules, being electrically charged or hydrophilic at one end, and neutral or hydrophobic on the other end, is responsible for a thermodynamic equilibrium which is represented by bilayer arrangement. Thus, whenever placed in an aqueous environment, phospholipids will tend to form bilayers.
This invention is a demonstration apparatus to be used in the teaching of biomembrane structure and fluidity. To my knowledge, it is original in its entirety; no such device has been built or suggested in any literature. The biomembrane demonstration apparatus consists of a transparent vessel containing two or three layers of immiscible fluids with sufficient difference in density to support layering. Each phospholipid molecule is represented by a spherical or polygonal head region (charged polar group) to which one or two slender tail-like structures (hydrocarbon chains) are attached. The model phospholipid molecules which form the lower lipid layer are constructed such that the head was a greater density than the fluid in the middle layer and the tail or tails have a lower density than said fluid in the middle layer, the combined density being intermediate between the densities of the bottom and middle fluid layers; with these conditions met, the heads float at the interface between said bottom and middle fluid layers, their tails pointing upward into said middle fluid layer. Similarly, the model phospholipid molecules which form the upper layer have a head density lower than the middle fluid layer and tails of a density greater than said middle fluid layer; the combined density of head and tails being intermediate between middle and upper fluid layers; with these conditions met, the heads float at the interface between said middle and upper fluid layers, their tails pointing downward into said middle fluid layer. The structures representing protein molecules are made of a compact or elongated shape; the density distribution within said model protein molecule and the overall density being adjusted such that said model protein molecule floats within either lipid layer or extends through both lipid layers.
One suggested form of the biomembrane demonstration apparatus consists of a small glass aquarium, a bottom layer of water, a middle layer of mineral oil and a top layer of air. The model phospholipid heads are made of plastic beads for the lower layer and of polystyrene spheres for the upper layer. The model phospholipid tails consist of sealed microtubing for the lower layer and insulated copper wire for the upper layer. Model protein molecules are constructed of segments of polyvinylchloride pipe, matching rubber leg tips and iron washers for weight and density adjustment.
When a mixture of described phospholipid and protein model molecules is poured into the vessel containing the layered immiscible fluids, said model molecules upon gentle agitation seek their proper position and result in a representative "Fluid Mosaic Membrane" bilayer just as thermodynamic law would dictate in the real biomembrane. Model proteins can be moved laterally within the model lipid bilayer without interrupting the continuity of said bilayer. Thus the invention allows vivid visualization of the structure and fluidity of biomembranes.